Recently a thin and light lithium polymer battery with a high energy density draws attention, and the development of the battery is proceeded to obtain a higher capacity. Many of lithium polymer batteries contain P(VDF-HFP) capable of retaining an electrolyte in the positive electrodes, negative electrodes and separators. By retaining the electrolyte in P(VDF-HFP), the conductivity of lithium ion is secured. As described above, the lithium polymer battery has the electrolyte retained in the polymer, and has less free electrolyte than in the case of a normal lithium ion battery. Therefore, the lithium polymer battery is expected as a battery being more excellent in safety than a normal lithium ion battery.
However, when the battery is in the overcharged state at a high temperature by a malfunction of a protection circuit, which is installed on a battery charger, a battery package or the like, even the lithium polymer battery excellent in safety possibly may cause the temperature of the battery to rise abnormally. Since the lithium ions in a lithium-containing positive active material extremely decrease and excess lithium ions are supplied to a negative active material, the battery in the overcharged state is thermally unstable. Especially, when the battery is exposed to a temperature around 80° C. in the overcharged state, the exothermic reaction occurs between the positive active material and negative active material, and the electrolyte, both in the active state, and the rate of the reaction is accelerated rapidly. This causes a phenomenon of, what is called, a thermal overdrive to occur to increase the temperature of the battery rapidly.
For example, there is disclosed in Japanese Unexamined Patent Publication No. hei 12-058065 an improvement in safety of the lithium polymer battery. This publication proposes to apply a porous polymer such as polyvinylidene fluoride (hereinafter, referred to as PVDF) having the shutdown function on the surface of at least one of the positive electrode and the negative electrode. In this case, the interruption of the current occurs at a high temperature (100-110° C.) by an operation of the shutdown function. As a result, the rise of the temperature of the battery is suppressed and the safety of the battery is improved. However, there is a problem that the energy density of the battery decreases if the PVDF is further added into the polymer battery containing P(VDF-HFP). Then, if a part of P(VDF-HFP) is replaced with PVDF, free electrolyte increases and, therefore, the advantage of the polymer battery decreases.
Here, the shutdown function means the function of the separator as described below. Namely, in the non-aqueous electrolyte secondary battery, when the short-circuit occurs in the battery or the battery is overcharged at a large amount of current, the temperature of the battery rises by heat generated in the battery. At that time, the pores of the porous separator are closed by softening or melting of the separator. This causes the insulating membrane to be formed and the internal resistance of the battery to increase and, then, the current is shut down to prevent the battery from generating heat.
On the other hand, many additives have recently been proposed as a disclosure for an improvement in safety of the lithium ion battery which dose not contain P(VDF-HFP) in the separator. For example, Japanese Patent No. 2983205 proposes to add an ether derivative to the electrolyte. The ether derivative produces a polymer when the battery is overcharged. Since this works as the resistor, the safety of the battery is improved in the overcharged state.
However, even if the ether derivative is added to the lithium polymer battery containing P(VDF-HFP) in the separator, this is not enough to secure the safety of the battery in the overcharged state. Especially, there is a problem that when the polymer battery in the overcharged state is exposed to a high temperature, the phenomenon of the thermal overdrive tends to occur. This is presumably ascribed to the facts that P(VDF-HFP) has a property to swell by absorbing the electrolyte unlike the polymer such as PVDF and the swelling is promoted especially at a high temperature, and that conventionally an appropriate combination of the ether derivative and the electrolyte has not been found out.
First, if the porous separator comprising P(VDF-HFP) swells by absorbing the electrolyte, the pores of the separator are closed by the electrolyte. In this case, the separator changes by itself to have a condition having the ion conductivity, which is a preferable condition for performing normal charge and discharge, and the movement of lithium ions is promoted. Moreover the higher the temperature becomes, the more easily the movement of the lithium ions is promoted. At the same time, however, the shutdown function does not operate for the separator in the swollen state with the absorbed electrolyte. Therefore, in the case of the lithium polymer battery using the separator comprising P(VDF-HFP), it cannot fully suppress the thermal overdrive occurring when the battery is exposed to a high temperature in the overcharged state.
Also, in the lithium ion battery and the lithium polymer battery, it is conventional to use the electrolyte containing ethylene carbonate (EC), dimethyl carbonate (DMC) and ethylmethyl carbonate (EMC) as the solvent. However, when the combination of the ether derivative and the electrolyte is not appropriate, it is possible that the polymerization of the ether derivative is suppressed and the ether derivative inhibits the shutdown function of the P(VDF-HFP).